Final answer:
The property illustrated in Step 1 of the expression manipulation is the associative property, which allows changing the grouping of terms in addition or multiplication.
Step-by-step explanation:
The property illustrated in Step 1 of the expression manipulation is the associative property. This property allows us to change the grouping of the terms in an addition or multiplication without changing the result. In the expression (12 + 10x - 12 - 6x + 6), the terms 12 + (10x - 12) have been grouped together, demonstrating the use of the associative property of addition. This property does not affect the sum or product of the terms; it just associates different terms together.